1. Field of the Invention
The present invention relates to a variable compression ratio device, and more particularly, to a variable compression ratio device capable of varying a compression ratio by varying a protrusion height of an inner piston which is coupled to an outer piston so as to move in an axial direction, and an internal combustion engine including the same.
2. Description of Related Art
In general, a compression ratio of an internal combustion engine is represented by a ratio of a maximum volume of a combustion chamber before compression to a minimum volume of the combustion chamber after compression during a compression stroke of the internal combustion engine.
When the compression ratio of the internal combustion engine is increased, the output of the internal combustion engine increases. However, when the compression ratio of the internal combustion engine is excessively high, a so-called knocking may occur to reduce the output of the internal combustion engine. Furthermore, the internal combustion engine may be overheated or a valve or piston trouble may occur in the internal combustion engine.
Thus, the compression ratio of the internal combustion engine is set to a specific value within a proper range before a knocking occurs. When the compression ratio is properly varied according to a load of the internal combustion engine, the fuel efficiency and output of the internal combustion engine may be improved. Therefore, various methods for varying the compression ratio of the internal combustion engine have been proposed.
The methods for varying the compression ratio of the internal combustion engine may include methods for varying the volume of a combustion chamber during a compression stroke.
For example, the level of top dead center of the piston may be varied during the compression stroke, or the volume of a secondary combustion chamber provided in a cylinder head may be increased or decreased.
U.S. Pat. No. 7,284,512 has disclosed a double piston structure having an outer piston and an inner piston, and the outer piston varies a compression ratio while moved vertically by a hydraulic actuator.
The variable compression ratio device having the above-described double piston structure requires a structural improvement for response and durability.
The information disclosed in this Background of the Invention section is only for enhancement of understanding of the general background of the invention and should not be taken as an acknowledgement or any form of suggestion that this information forms the prior art already known to a person skilled in the art.